Using LimitedExternal Query Processors

When answering queries using external information sources, their contents can be described by views. To answer a query, we must rewrite it using the set of views presented by the sources. When the external information sources also have the ability to answer some (perhaps limited) sets of queries that require performing operations on their data, the set of views presented by the source may be innnite (albeit encoded in some nite fashion). Previous work on answering queries using views has only considered the case where the set of views is nite. In order to exploit the ability of information sources to answer more complex queries, we consider the problem of answering conjunctive queries using innnite sets of conjunctive views. Our rst result is that an innnite set of conjunctive views can be partitioned into a nite number of equivalence classes, such that picking one view from every nonempty class is suucient to determine whether the query can be answered using the views. Second, we show how to compute the set of equivalence classes for sets of conjunctive views encoded by a datalog program. Furthermore, we extend our results to the case when the query and the views use the built-in predicates <, , =, and 6 =, and they are interpreted over a dense domain. Finally, we extend our results to conjunctive queries and views with the built-in predicates <, , and = interpreted over the integers. In doing so we present a result of independent interest, namely, an algorithm to minimize such queries.